The present invention relates to a trellis-coded modulation (TCM) system, and more particularly, to an apparatus which generates a coded signal which can correct errors produced during a magnetic recording and reproducing of a signal and converts the coded signal into digital data having the corresponding amplitude and phase.
An early TCM system is described in "Trellis-coded Modulation with Redundant Signal Sets, Part 1: Introduction" by Gottfried Ungerboeck (IEEE Communications Magazine. February 1987. pp 5-21).
FIG. 1 is a block diagram of the encoder for a non-linear eight-state TCM code having a 32-CROSS signal set. The encoder is constituted by a data source 110, a convolutional encoder 120, a subset selector 130, a signal point selector 140, and a modulator 150.
In FIG. 1, the convolutional encoder 120 outputs 3-bit data 103 by adding a redundant bit to 2-bit data 102 among the data transmitted from the data source 110. The output 3-bit data 103 selects a subset in the 32-CROSS signal constellation shown in FIG. 2, among eight subsets which are set-partitioned by the Ungerboeck method, and outputs the selected subset to the signal point selector 140. At this time, the subset includes four signal points. The signal point to be transmitted by an uncoded 2-bit 101 signal is selected by selector 140 in the subset and is transmitted to the modulator 150. The outputs of the signal point selector 140 are rearranged by designating an amplitude and phase to each symbol by the set partition. The values of the rearranged signal points are converted to level values by the modulator 150 and transmitted to channels on a carrier. In the coding method using TCM modulation, if the bandwidth is restricted because input data is encoded to maximize the euclidean distance between symbols on a signal space, a higher coding gain than that of a prior art error correction system can be obtained without increasing bandwidth.
However, since the processes of selecting a subset in the signal constellation and selecting a signal point in the subset are separated from each other, the embodiment and integration of such a circuit is open to improvement.